Neurodegenerative disease such as Alzheimer's disease (AD) and Parkinson's disease (PD) are prevalent in the elderly population and the number of patients is increasing exponentially with the aging of society. Moreover, reports of early-onset types of neurodegenerative disease in the young are not uncommon. Thus, there is great interest in developing treatments that help stop the progress of the disease or recover damaged brain tissues.
The exact causes of such neurodegenerative disease have not been established yet. According to what is known so far, neuronal cells in specific locations in the brain (e.g. the hippocampus or substantia nigra) are damaged leading to a defective neural network among the reduced number of neuronal cells, which results in various symptoms of the neurodegenerative disease.
Research is being carried out in various fields to look for treatments. To date, drugs related to the relief of symptoms include memantine (NMDA receptor antagonist), L-DOPA (dopamine mimic drug), etc. Other drugs are also limited to a short-term effect or have been found to have side effects with continual use, making it difficult to expect them to provide for treatment beyond the temporary relief of symptoms. Therefore, a fundamental treatment for the cause of neurodegenerative disease is in great need.
Neural stem cells (NSC) and neural progenitor cells (NPC), cells that are capable of differentiating into neural cells, are present in the adult brain. Neural stem cells are present in the subventricular zone of the lateral ventricle and dentate gyrus of the hippocampus, and it is in this region that neurogenesis occurs throughout the entire animal's life through differentiation and proliferation of neural stem cells (Zhao et al. (2008) Mechanisms and Functional Implications of Adult Neurogenesis. Cell 132:645-660).
Since brain neuronal cell damage and loss occur in neurodegenerative disease, replacement of damaged or lost neurons with normal functioning neurons through the stimulation of NSCs and NPCs could be a fundamental treatment for neurodegenerative disease. This method of treatment includes the method of stem cell treatment, where NSCs and NPCs are isolated from the patient's body, stimulated in vitro to differentiate into neurons, and then transplanted back into the patients. However, there is difficulty in isolating NSCs and NPCs from patients and then transplanting them back into patients. Also, the transplanted NSCs and NPCs quickly lose their activity in the brain, requiring repeated transplantations. As an alternative, instead of transplanting neural stem cells into patients, a method of generating neurons in the patient's brain by stimulating NSCs and NPCs to differentiate with the use of drugs has recently been proposed (Davies et al. (2015) Stemistry: The Control of Stem Cells in Situ Using Chemistry. J. Med. Chem. 58:2863-2894).
Amyloid-beta (Aβ) is a peptide of 36-43 amino acids, which is produced by the cleavage of the amyloid precursor protein (APP), a type 1 integral membrane protein, by β-secretase and γ-secretase. Amyloid-beta (Aβ) aggregates as soluble amyloid-beta (Aβ) oligomers and then, via protofibrils, forms insoluble Aβ fibrils to eventually accumulate as amyloid plaques in the brain. The deposition of Aβ in the brain is associated with synapse damage, neuronal damage, and brain atrophy and ultimately results in damage in memory and cognitive functions, two very typical symptoms of Alzheimer's disease (AD). Among the various forms of Aβ, soluble Aβ oligomers, especially trimers and tetramers, are thought to be the most toxic forms of Aβ that are associated with neuronal dysfunction and synaptic damage (Murakami, (2014) Conformation-specific antibodies to target amyloid β oligomers and their application to immunotherapy for Alzheimer's disease. Biosci. Biotechnol. Biochem. 78(8):1293-1305; Jana et al. (2016) Membrane-bound tetramer and trimer Aβ oligomeric species correlate with toxicity towards cultured neurons. J Neurochem. 136(3):594-608).
Therefore, protecting neurons from Aβ, especially the oligomeric forms of Aβ, is considered the potential target for AD treatment. However, AD patients have already undergone significant neuronal damage, so in addition to neuroprotection, neuro-regeneration through the differentiation of endogenous neural stem cells is required for the fundamental treatment of AD.
MEK (mitogen-activated protein kinase kinase; also known as MAP2K or MAPKK) is a member of the MAP kinase (mitogen-activated protein kinase; MAPK) signal transduction pathway (written herein as ‘MAPK/ERK pathway’) that follows in the sequence of Ras-Raf-MEK-ERK. When various signaling molecules such as growth factors, hormones, cytokines, etc., bind to cell membrane receptors and activate receptor tyrosine kinase, the protein Ras GTPase is activated, which results in the recruitment of cytoplasmic Raf to the cell membrane. Activated Raf phosphorylates and activates MEK and ERK, sequentially, and activated ERK in turn translocates into the nucleus to activate various transcription factors. These transcription factors then bind to the promoters of various genes to control cell proliferation, differentiation, and survival. Because the MAPK/ERK signal transduction pathway is hyperactivated in tumor cells, the kinases were seen as important targets to inhibit the disease progress in cancer and other proliferative disease.
There are 7 proteins (MEK1-MEK7) known to be in the MEK family and of these, only MEK1 and MEK2 are involved in the signal transduction of the Ras-Raf-MEK-ERK pathway. Although MEK1 and MEK2 are encoded by different genes, they share high homology (80%) both within the C-terminal catalytic kinase domains and most of the N-terminal regulatory regions. Although oncogenic forms of MEK1 and MEK2 have not been found in human cancers, it is known that constitutive activation of MEK has been shown to result in cellular transformation. In addition, MEK can also be activated by other oncogenes. Accordingly, the inhibition of MEK1 and MEK2 has been studied as a target for anticancer drug development. It is unclear, however, what role MEK1 and MEK2 and the MAPK/ERK pathway have on the proliferation and differentiation of adult neural stem cells.
Furthermore, there are study results that indicate a link between the MAPK/ERK pathway and Aβ or tau proteins in the brain of Alzheimer's (AD) patients. Unfortunately, it is unclear whether AD treatment will require the activation or the inhibition of this signaling pathway and whether or not the control of this signal transduction pathway can be linked to AD treatment.
There are reports of increased levels of expression of proteins in the MAPK/ERK pathway in patients of very early stage Alzheimer's disease (Arendt et al. (1995) Increased Expression and Subcellular Translocation of the Mitogen-Activated Protein Kinase Kinase and Mitogen-Activated Protein Kinase in Alzheimer's Disease. Neuroscience 68(1):5-18; Gartner et al. (1999) Elevated Expression of p21ras is an Early Event in Alzheimer's Disease and Precedes Neurofibrillary Degeneration. Neuroscience 91(1); 1-5), of the association between ERK1/2 and MEK1/2 with the hyperphosphorylation of tau in Alzheimer's patient brain (Pei et al. (2002) Up-Regulation of Mitogen-Activated Protein Kinases ERK1/2 and MEK1/2 is Associated with the Progression of Neurofibrillary Degeneration in Alzheimer's Disease. Brain Res Mol Brain Res. 109(1-2):45-55), and of increased Ras expression and ERK1/2 activation in B103 cells (mouse neuroblastoma cells) expressing amyloid precursor protein (APP) (Chaput et al. (2012) SILAC-based Proteomic Analysis to Investigated the Impact of Amyloid Precursor Protein Expression in Neuronal-Like B103 Cells. Electrophoresis 33(24):3728-3737).
Meanwhile, other studies have shown that when ERK1/2 is activated, apoptosis induced by Aβ is inhibited and Aβ accumulation decreases (Guerra et al. (2004) Plasma Membrane Oestrogen Receptor Mediates Neuroprotection Against β-Amyloid Toxicity Through Activation of Raf-1/MEK/ERK Cascade in Septal-Derived Cholinergic SN56 Cells. J. Neurochem. 91:99-109; Watson et al. (2005) Macrophage Inflammatory Through Activation of Mitogen-Activated Protein Kinase an Phosphatidylinositol 3-Kinase Signaling Pathway. Molecular Pharmacology 67(3):757-765; Mills et al. (1997) Regulation of Amyloid Precursor Protein Catabolism Involves the Mitogen-Activated Protein Kinase Signal Transduction Pathway. J. Neurosci. 17:9415-9422). In addition, it has been reported that ERK1/2 decreases the activity of γ-secretase, which produces Aβ from APP, and decreases the expression and activity of BACE1(β-secretase1) in oxidative stress conditions (Tamagno et al. (2009) JNK and ERK1/2 Pathways Have a Dual Opposite Effect on the Expression of BACE1. Neurobiology of Aging 30:1563-1573).
Therefore, there are conflicting views on the roles of the MAPK/ERK pathway regulation and MEK inhibition in relation to neurodegenerative diseases, such as Alzheimer's disease, which have yet to be clearly elucidated.